The investigator proposes to evaluate the role of opioid neurons in the age-and stress-related alterations in physiological process. Both in vivo and in vitro indices of opioid neuronal function will be used. The in vivo indices, alterations in analgesic response and cross-tolerance to morphine, will be used to analyze activation of opioid systems during chronic exposure to stress. In vitro indices of endogenous opioid activity will employ the measurement of met- and leu-enkephalin content in several brain and spinal cord regions, opioid binding sites in these same areas, and pituitary and plasma concentrations of beta-endorphin. These indices of aging will be evaluated (as part of the core project) after 7 months of exposure to the "stressor" when stress is initiated at 4, 11, or 18 months of age. The effects of alterations in the length of exposure to chronic stress on the rate of aging of opioid functions will be evaluated.